The present invention is related to pressure detecting apparatus, i.e., a pressure sensor arranged in such a manner that a pressure sensitive element is provided inside a housing containing an elongate-shaped pipe portion which is projected from one edge side of the housing.
This sort of pressure detecting apparatus is arranged by employing a housing, an elongate-shaped pipe portion, and a pressure sensitive element, the apparatus which is disclosed, for instance, in Japanese Laid-open Patent Application No. H05-34231. The elongate-shaped pipe portion is provided in such a manner that the pipe portion is projected from one edge side of the housing. The pressure sensing element is provided in the housing and outputs a signal in response to detected pressure.
In the above-described pressure detecting apparatus, the pipe portion of the housing is inserted into an object to be detected, the pressure of which will be detected, and then, pressure to be detected (namely, detecting pressure) derived from the object to be detected is applied to a tip portion of the pipe portion. Then, this detecting pressure is applied to the pressure sensitive element provided in the housing so as to detect the pressure.
In the conventional pressure detecting apparatus, while a diaphragm for receiving the detecting pressure is provided on the tip portion of the pipe portion, and a pressure transferring member having a rod shape (referred to as “rod”), the detecting pressure may be transferred from the diaphragm via the rod to the pressure sensitive element.
Such a pressure detecting apparatus equipped with the rod functioning as the rod-shaped pressure transferring member is applied to a combustion pressure sensor, and the like. The combustion pressure sensor detects pressure (namely, internal cylinder pressure) in a combustion chamber of an engine. In this case, the pipe portion of the housing is inserted into a mounting hole of an engine block which corresponds to the object to be detected. As a result, the length of the rod must be elongated in response to the length of the pipe portion, namely, a transfer distance of pressure.
However, as explained above, if the length of the rod is made longer, then the weight of this rod is increased. As a result, a resonant frequency of the rod is superimposed with a vibration frequency of knocking of combustion, so that a resonant phenomenon may occur in the rod. Accordingly, such a problem may occur with respect to a sensor characteristic. That is, knocking signals are embedded in noise which is produced by the resonant phenomenon of the rod, so that the knocking signals cannot be measured.
Also, in the case that the length of the rod is made long, since the rod itself may be easily deformed, contact conditions between the rod and either the diaphragm or the pressure sensitive element are changed. If such a change in the contact conditions happens to occur, then precision in pressure transfer characteristics is deteriorated, which may give an advance influence to the sensor characteristic.
Further, in the case of such a pressure detecting apparatus that a pressure sensitive element has been connected to a flexible printed-circuit board by way of a flip flop joint, if a cooling/heating cycle is repeatedly carried out, then a large amount of stresses may be produced in the bump joint portions, because of differences in thermal expansion coefficients of these structural elements, in particular, a difference between a thermal expansion coefficient as to the pressure sensitive element made of silicon, and another thermal expansion coefficient as to the flexible printed-circuit board made of resin.
Also, while the sensing unit repeatedly receives pressure by the detected pressure, bumps are depressed by this pressure, so that a large amount of stresses may be produced in the bump joint portions. Then, in the bump joint portions which have received the stresses, there is such a risk that a wire disconnection may eventually occur.
Furthermore, if the flexible printed-circuit board is bent in such a manner that the bending angle “θ” becomes smaller than, or equal to 90 degrees, then such a stress applied to a direction along which one edge portion is stripped from the strain gauge is not substantially produced at one edge portion of the flexible printed-circuit board, namely at the joint portion to the strain gauge under this bending condition.
However, when the flexible printed-circuit board is bent under such a bending condition that the bending angle “θ” becomes smaller than, or equal to 90 degrees, the stress which is applied to the bending portion of the flexible printed-circuit board is large, so that a damage may readily occur. As a result, the following problems may probably occur. That is, cracks, bends, and breaks may be produced in a base, a copper foil, and the like, which constitute the flexible printed-circuit board.